適當的訂約方法或策略，使得專案執行有較好的績效。雖然有不少訂約方法的研究，但使用者對於其中的精神並未完全瞭解，造成不少工程專案執行困難。
本研究提出On-Call Contracting，待命訂約策略雖然很少出現在有關訂約的研究中，但已行之多年，它是由契約型態組合而發展出來的訂約策略，將複雜專案分成小的任務命令(TO)，讓工作變得較單純容易，也提供業主汰換績效不佳廠商的機制。
On-Call訂約是單一及多重契約的混合，它的特性包括規劃模式、涵蓋要求、契約類比、付款方法、風險分擔，其原理具有其他最佳實務的特性，如範圍凍結的完整規劃，同步工程的重疊進行，施工管理及施工性的運用。使用On-Call訂約必須要有足夠及有經驗的人，以執行增加的行政工作及TO規劃，否則會出現雙重訂約缺點。
On-Call訂約符合營建業需求，它可改善傳統發包方式之僵化，具有統包優點，亦降低統包在成本與品質的不確定性，因為專案的分割，使業主清楚掌握成本，以績效評估的方式激勵承包商投入專案的程度，對品質有積極的保證作用。
本研究亦藉契約型態、影響訂約因素、專案執行系統之比較，找出各種情況適用的訂約方法，供業主選擇採用，使訂約策略與管理更趨完善。

A proper contracting strategy results in better project performance. Although plenty of relative studies of contracting methods exist, the users just do not realize the essence of contracting that makes the contract execution more difficult.
On-Call contracting seldom appears in contracting studies, but it is an existing practice in the industry. The strategy, developed by combining different contract types, divides a complex project into task orders (TO), makes the work easier to define and provides a mechanism to substitute the under-performed contractor.
On-Call contracting combines single prime and multiple contracts. The characteristics of On-Call contracting include planning mode, requirements covered, contract analogy, payment method, and risk allocation. Its principles are derived from other best practices: complete planning from project scope freezing, overlapping from concurrent engineering, as well as design and construction managers from CM and constructability. Executing On-Call contracting requires more and experienced personnel for additional administrative work and TO planning. Otherwise, the disadvantages of its dual contracting strategies would appear.
On-Call contracting meets the needs of the construction industry. It improves the stiffness of traditional Design-Bid-Build method. Not only does it retain the advantages of the design-build, but also reduces the uncertainties of project cost and quality. By dividing the project into tasks, the owner can have a better idea about the cost. By evaluating contractors’ performance, the contractors are made more attentive to their work. Thus, the quality would also improve.
This research also compares contract types, contracting influence factors, and project delivery systems to identify better contracting strategies. It makes the contracting strategy and management more complete. The owners can select contracting strategies accordingly.

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