implemented in the request 21 ina second step 22 of the embodiment according fig2 the p cscf 3 returns to the msc 2 a timer value 22 which indicates the time left the ue 1 has on prior its expiration timer will expire the request of the first step 21 can be a subscription of the msc 2 to the p cscf 3 i e the p cscf 3 will inform the msc 2 about any changes this could happen e g if the coverage situation might have changed and may have allowed that the ue 1 updates the ims registration on its own this might be possible if the ue 1 is switched to a network which can handle a ps request or supports dtm the msc 2 can request the expiration timer value latest before it expires and while the cs call is still ongoing the msc 2 decides in a third step 23 of the msc registration expiration update procedure according the embodiment of fig2 to update the registration timer in the p cscf 3 prior to the expiry of the expiration timer in the ue 1 in a fourth step 24 of the embodiment according fig2 the msc 2 initiates a re registration to update the registration timer in the p cscf 3 to avoid that the p cscf 3 will request the msc 2 for authentication according to a normal re registration procedure to an ims network the msc 2 may indicate the re registration procedure as an msc initiated re registration procedure alternatively the msc 2 could send a special.