Recently we updated our FreePBX and since then just one person couldn't phone me, else everything is working ok.
After some investigation for what is going on, i found error 488 which usually is with incompatible codecs. But looking at that everything looks fine so that couldn't be the cause.
Googling for that error showed up a wiki about it:
"It can happen that Asterisk will return "488 Not acceptable here" if caller device asks for T38 support, but it is no enabled in Device settings."
"In this case option "T.38 Support" should be enabled on Device settings to allow call go through."
Strange that this is needed now after we updated FreePBX, but to enable T.38, add following line into sip.conf in context [general] or sip_additional.conf in the users context:
t38pt_udptl = yes
And don't forget to reload/restart asterisk! ("core reload" in asterisk console)