When Tom calls Eline for the first time when she is in quarantine, he uses the 5-digit phone number Eline gave him. This 5-digit number is an internal number in the hospital (regular phone numbers in Belgium having 9 or 10 digits).
Tom calls her using his mobile phone (not using the phone he has on his room that would allow for an internal number) and clearly uses only 5 digits. The connection cannot be established that way.