Iphone Cutting calls off desk phone when recieving TXT message alerts

Hi there

I am not sure this may be the correct place for this, but anyway.

I have a strange situation where a txt message alert on an Apple iPhone cuts of in-progress calls on a PABX system phone.

Incoming ringing no problem, just TXT alerts

Androids no problem

Any one got any ideas

It's got the PABX manufacturer scratching there heads.

Regards