If he is making a reimbursement to a person for any service/goods that is not chargeable to tax in India by virtue of Section 9 or in case it may be chargeable u/s 9 but over ridden by the DTAA then you do not have to deduct the TDS.
If the agent is being rimbursed for an expense where payee is providing you service that should have a TDS being done irrespective of whether the service provider has PE in India or not, you will have to deduct TDS. In such case it is better to pay grossed up TDS.
Income from Telephone service or Car rental is not taxable in India if provided and received abroad. If he is paying there to these providers it should be on your behalf and your expenditure during course of your business. The bill itself has to be in your name. It should not be his own expense that you are reimbursing to him. Therefore you will have to go to your CA and get form 15CB and he will analyse your transaction and arrangement with the agent and certify whether TDS is required or not. However, you are still liable to pay Service Tax on these services if used for your business in India. It is better you subscribe to calling card here in India itself with validity in his country and give him the T PIN to use it. Do not reimburse for telephone at his office that would be his expense during course of his business.