I have the same with v.188.8.131.52. It is a right pigs ear, which is what you would expect from a Microsoft company. Having said that, I get the identical problem with my Skype phone on the 3 network. Both will not sign in about 30% of the time, so despite the person the other end having their software up and running, you can't reach them.
The answer to my Skype mobile is the same as the PC ó Shut down the software and restart, which seems to point to a system bug rather than users firmware version, though they haven't yet cured the error reporting bug, where the Skype site tells you you have no Internet access.
All of the above is of no use to me when trying to phone home to check on a blind and severely disabled wife, who cannot restart the software.
My answer when I go out and about is to also leave PhonerLite running on the PC at home. It also does auto-answer. So when loaded with my free Sipgate number, it is a backup where I can dial in for 5p per minute from my mobile, something you cannot do with Skype, even when paying £11.50 for three months Skypein Number.