I've found that the majority of the time if you can't get through to an escort they will return your missed call.
Just a little patience required.
I know it's hard to wait when the mood takes you but if you want the escort you've spent 10mins+ looking for on the EI website its worth the wait - don't panic book some-one else.
As they say some things are worth waiting for.
Also make sure you don't call on private number as Escorts will not answer those calls ..... those type of calls will ring out and go to VM which is very understandable.