Note: This question relates to the portable Iridium 95XX phones. The Fixed Install Iridium phones are likely to have been supplied an external antenna
The standard answer is that: "yes an external Iridium antenna is required whenever you are operating an Iridium phone indoors / undercover"...
...However... MailASail cautiously suggest that an external Iridium antenna is highly desirable, but not always 100% "necessary". Budget constraint is the largest reason for not taking an externaly antenna. However, it does give a dramatically improved signal on the portable phones, even compared with using the phone outside with a good view of the sky (with the exception of the new Iridium 9555, the antenna built into the older 950X phones does not seem to be as sensitive as the external antenna)
If you will be using your Iridium phone for more than "emergency use", which means more often that once a week or so, then you definitely want to put the external Iridium antenna on your "Wish List". You will experience fewer call drops and as the section below on "signal vs speed" shows you will benefit from spending less time online, which means lower call costs and offsets much of the price of the marine antenna.
Of course as long as you are prepared to hold the phone outside when you use it, watch the signal level rise/fall and time your calls at maximum signal then it's very possible to get good service without the external antenna. However, please do not underestimate just how important a good strong signal is to getting satisfactory Iridium performance.
Approximately 70% of our customers now take a proper Iridium marine antenna (this is also a function of an increasing number of people who go for the robust "Iridium 9522 compact fixed install" units)
So if you are on a "plastic" or wood boat then we would say an "external Iridium antenna is highly desirable, but let the equipment budget be the ultimate decider". Of course if you have a metal boat (or in a car or building) then an external antenna is usually mandatory...
Signal Strength vs Effective Download "Speed"
You need 5 bars of signal strength (as shown on the phone display) to get satisfactory email download speeds. At 4 bars your download speed is perhaps 1/2 normal rate, and at 3 bars it's perhaps 1/10 normal speed, and at 2 bars or less it will probably drop the call... The performance really drops off a cliff as the signal strength/quality drops, so you need to be seeing 5 bars signal strength for several mins on end in order to use the satellite phone *without frustration*
You really should have an external antenna whenever you are operating an Iridium phone anywhere except with obstructed views of the whole sky. If you want push button, always there, reliability with no messing around then get an external antenna.
However, if you are on a budget and prepared to mess around a little and reposition the computer and phone and wait a while to try and get the best signal then you may be happy using only the standard antenna on the phone (or the car antenna included with the phone)