Learn the phonetic alphabet and use it when you are confirming spellings back. It’s not difficult to pick up and N for November sounds so much better than ‘So that’s N for Noddy?’
Even if you have no idea what they are talking about remain positive and professional by using ‘let me find that out for you and come back to you’ rather than ‘I don’t know’ as it shows interest and a willingness to help.
If possible give the caller an idea when they can expect a call back. If your colleague is in a meeting or out of the office for the afternoon explain it may be the following day that they get a response and check if someone else can help them. Your poor colleague may well get a frosty reception if they call back a day later and from the way you took the message the caller had expected a quicker response.
If you have to put people on hold make sure you go back to update them every few seconds or so and remember to keep offering to have them called back